This invention relates to thermal imaging devices and in particular to thermal imaging devices comprising an array of pyroelectric detector elements responsive to infra-red radiation.
the main factor limiting the performance of existing thermal imaging devices is the thermal conductance between adjacent detector elements and between each detector element and the supporting and interrogating structure.
In our copending European Patent Application Nos. 89305494 and 89305496 there are described thermal imaging devices which are designed in such a way as to reduce the thermal conductance between the detector elements and the supporting and interrogating structure. These devices will now be described with reference to FIGS. 1 to 3 and 4 and 5 of the accompanying figures in which:
FIG. 1 shows a plan view of part of the electrode structure of the device described in European Patent Application No. 89305494;
FIG. 2 shows a plan view of part of a surface of the device shown in FIG. 1, and
FIG. 3 shows a section along the line A--A' of the device of FIGS. 1 and 2;
FIG. 4 shows a plan view of part of a surface of a device described in European Patent Application No. 89305496; and
FIG. 5 shows a section along the line B--B' of the device of FIG. 4. Referring to FIGS. 1 to 5 in which corresponding features are correspondingly labelled a pyroelectric film 1 has an array of interconnected electrodes 2 formed on one side and an array of discrete electrodes 3 formed on the other side. Electrically conductive pads 4 are connected by long thin conductors 5 to the discrete electrodes 3 and by electrically conductive pillars 6 to input pads 7 of an integrated signal processing circuit located adjacent to the input pads of electrical circuits formed in regions 8 of a silicon wafer 9. The input pads 7 are separated by regions 10 of a passivating layer of an insulating material, for example silicon monoxide. In the arrangement of FIGS. 1 to 3 each discrete electrode 3, as best seen in FIG. 1, is square shaped with a central gap 11 in which the pad 4 lies. The long, narrow conductor 5 extends around the pad 4 within the gap 11. The interconnected electrodes 2, as best seen in FIG. 2 each have a central hole 12 corresponding to the gap 11 in the respective discrete electrode 3 and may carry an infra-red absorption layer 13, consisting of, for example, black gold, platinum black or a metal-dielectric-metal sandwich structure.
Referring now to FIGS. 4 and 5, in the device illustrated in these figures the discrete electrodes 3 are spaced from respective electrically conductive pads 4 and connected to them by long narrow conductors 5. The pillars are attached to respective electrically conductive pads 4, each of which lies centrally disposed with respect to several (in the case illustrated four) adjacent discrete electrodes 3. The interconnected electrodes 2 are in positions corresponding to the discrete electrodes 3 and again carry an appropriate infra-red absorption layer 13.
Both arrangements shown in FIGS. 1 to 5 provide a low, thermal conductance from the active areas of the discrete electrodes 3 to the pillars 6 because the electrical connection between the electrodes 3 and pillars 6 includes the long narrow conductor 5. However, both arrangements involve significant areas either within (in the device illustrated in FIGS. 1-3) or around (in the device illustrated in FIGS. 4 and 5) the discrete electrodes 3 where the incident radiation is wasted and consequently the signal to noise ratio of the device is reduced. In the device illustrated in FIGS. 4 and 5 the discrete electrodes 3 could be brought closer together by meandering the conductor 5, as proposed in European Patent Application No. 89305496 but there would still be significant incident radiation wasted at and around the pads 4.
It is an object of the present invention to provide a thermal imaging device of the type described in our copending European Patent Application Nos. 89305494 and 89305496 but wherein substantially the whole area of the pyroelectric film may be utilized to produce signals.